KR100818681B1 - METHOD OF REDUCING NOx IN DIESEL ENGINE EXHAUST - Google Patents

Abstract

Reduction of nitrogen oxides in the oxygen-containing exhaust stream 12 using ethanol 26 as a reducing agent for selective catalytic reduction using plasma. Exhaust gas 12 generated from a diesel engine or other lean-burn power source 10 comprises nitrogen oxides, in particular NO. Ozone generated from the plasma reactor 20 is injected into the exhaust stream 16 to convert NO into NO ₂ followed by the addition of ethanol 26 after the plasma injection. The NO _x is then reduced by contacting the exhaust stream 28 with a dual-bed catalyst reactor 24 comprising BaY (or NaY) and CuY zeolite catalysts in which ethanol is present as the reducing agent. The plasma power density and molar ratio of ethanol to NO _x fed to the catalytic reactor 24 is adjusted as a function of catalyst temperature for optimal performance of the plasma-catalyst system. An average conversion of NO _x to N ₂ of at least 90% is achieved in the temperature range of 200 to 400 ° C.

Diesel Engine, Nitrogen Oxide, Reduction, Ethanol, Catalyst, Exhaust Gas

Description

Reduction of NO in diesel engine exhaust gas {METHOD OF REDUCING NOx IN DIESEL ENGINE EXHAUST}

The present invention relates generally to the treatment of exhaust gases from a power source to which hydrocarbon fuels operated with a fuel lean combustion mixture, such as a diesel engine, are replenished. More specifically, the present invention is directed to treating the NO _x content of exhaust gases with separately added ethanol and plasma treated air before the exhaust gases are contacted and passed through a base metal-exchanged zeolite catalyst. It is about.

Diesel engines, some gasoline-fueled engines, and many hydrocarbon-powered power plants operate at higher than stoichiometric air to fuel mass ratios for improved fuel economy. However, such lean-burning engines and other power sources produce hot exhaust gases with relatively high contents of oxygen and nitrogen oxides (NO _x ). For diesel engines, the temperature of exhaust gases from warm-up engines is typically in the range of 200-400 ° C., in volume percent, about 17% oxygen, 3% carbon dioxide, 0.1% carbon monoxide, 180 ppm hydrocarbons, 235 ppm NO _x and the usual composition of the remaining nitrogen and water.

These NO _x gases, typically including NO and NO ₂ , are difficult to reduce to nitrogen because of the high oxygen content in the hot exhaust stream. It is therefore an object of the present invention to provide an improved method of reducing NO _x in such gas mixtures.

The present invention provides a method for reducing NO _x in an exhaust stream using any nonmetal-exchanged zeolite Y catalyst with ozone and ethanol added upstream. The present invention is also useful for reducing NO _x in hot gas streams containing large amounts of oxygen. The present invention is useful for treating exhaust streams from lean-burn power plants and engines, such as diesel engines. This will be illustrated by applying this to a diesel engine.

In the present invention, the NO _x containing exhaust is ultimately passed through or via a dual bed catalyst wherein the upstream bed is sodium Y zeolite or barium Y zeolite and the downstream bed is copper Y zeolite. Such nonmetal-exchanged zeolite catalysts will sometimes be referred to herein as NaY, BaY or CuY, respectively. The effect of the dual bed catalyst is firstly by adding upstream to the plasma exhaust gas from which ozone is released from air to convert NO into NO ₂ , and secondly to help reduce NO ₂ to N ₂ . Promoted by separating and adding ethanol to the exhaust gas. While some ethanol is oxidized to acetaldehyde, a plasma reactor is used at lower catalyst temperatures in order for the ozone to convert NO to NO ₂ . The power density requirement of the plasma reactor is a function of the temperature of the dual bed catalytic reactor (or the temperature of the exhaust gas closely related at the reactor inlet). In order to reduce the NO ₂ content of the exhaust gas, the ethanol is added in a sufficient amount based on the temperature at the inlet of the dual bed catalyst or inside the reactor. Since the exhaust gas then passes through the dual bed catalyst, some ethanol is oxidized to acetaldehyde, and the ethanol and acetaldehyde participate in the reduction of NO ₂ to N ₂ . The ethanol and acetaldehyde were found to be effective in reducing NO ₂ on the catalyst surface without excessively lowering catalyst activity.

Ozone is properly released by passing air through a highly efficient non-thermal plasma generator. In a preferred embodiment, the plasma generator is a tube having a dielectric cylindrical wall that defines a reactor space. A straight high voltage electrode is disposed along the axis of the tube in this reactor space. An external ground electrode made of electrically conductive wire spirally wraps around the cylindrical dielectric wall. When a high frequency AC voltage is applied to the center electrode, plasma is formed in the air passing through the reactor. The combination of the spiral ground electrode and the linear axial electrode forms a spiral region in which the active and passive electric fields are entangled with each other. Importantly, ozone and possibly other chemically activated oxygen ions or radicals and other species are formed in the air stream flowing through the plasma reactor. As mentioned above, the output of the plasma reactor is introduced into the exhaust stream and the ozone oxidizes NO to NO ₂ . The NO ₂ downstream is reduced to N ₂ by reacting with ethanol and acetaldehyde on the dual bed nonmetal-exchanged zeolite catalyst.

The temperature of the exhaust gas (or the temperature of the bed itself) introduced into the dual reduction catalyst bed is monitored and the NO _x content of the exhaust gas is measured or evaluated. As described in more detail below, the plasma power density and the EtOH / NO _x molar ratio are both set and / or adjusted based on this reduction catalyst temperature. In general, the plasma power demand decreases as the reduction catalyst temperature increases, while the ethanol demand increases as the catalyst temperature increases. In a stationary power plant operating at steady state conditions, the catalytic reactor can operate at a stable temperature for a long period of time and the NO _x level can remain substantially the same. In this case, the plasma power density and EtOH / NO _x once set may not require frequent monitoring. However, in a vehicle diesel or lean burn gasoline engine, conditions during warming up and various driving conditions will require more frequent monitoring or measurement of the NO _x content of the exhaust gas and the catalyst temperature.

At a reduction catalyst temperature range of 200-400 ° C., the process of the present invention can achieve an average of 90% NO _x to N ₂ conversion over the extended operation of the dual bed nonmetal exchanged zeolite catalyst.

Exhaust gases from diesel engines contain incompletely burned hydrocarbons, in particular diesel particulates and carbon monoxide, which are preferably removed by catalytic oxidation and filtering of the exhaust gas before ozone is added to the exhaust gas.

Other objects and advantages of the present invention will become apparent from the following description of the preferred embodiments.

1 is a schematic flowchart of exhaust gas from a diesel engine illustrating a preferred method of NO _x reduction according to the present invention.

2 is a partial side cross-sectional view of a non-thermal plasma reactor used to treat air for ozone emission in the practice of the present invention.

3 is a function of the time of exposing the catalyst to the exhaust stream when using propylene (dash-dotted line), gasoline (solid line) or ethanol (dotted line) as reducing agent. , NO _x conversion efficiency graph on dual-bed nonmetal-exchanged Y zeolite catalytic reduction reactor.

4 shows a graph of the ethanol-to-NO _x feed ratio as a function of the reduction catalyst temperature as well as a graph of plasma energy density (J / L) versus reduction catalyst temperature.

5 is a graph of% NO _x conversion (%) versus reduction catalyst temperature over a range of 150-400 ° C. in a dual bed experimental reactor.

6 is a schematic flowchart of an experimental sidestream plasma-catalyst system in an engine dynamometer test.

7 is a graph showing the advantages of adding plasma treated air to the side stream. The solid line shows the NO _x conversion when the plasma is added, and the dotted line shows the NO _x conversion when the plasma is not added.

8 is a bar graph of NO _x conversion at a reduction catalyst inlet temperature of 200-300 ° C. The bar data shows NO _x in two cases: (a) ethanol is injected directly upstream of the plasma-added exhaust stream, and (b) ethanol is injected downstream of the plasma-added exhaust gas. Compare the transformations.

The practice of the invention is schematically illustrated in FIG. 1. Block 10 represents a diesel engine and line 12 represents the flow of exhaust gas from the diesel engine 10. Diesel engines typically operate at higher air-to-fuel mass ratios than stoichiometric air-to-fuel ratios, and the exhaust contains significant amounts of O ₂ as well as unreacted N ₂ (from air). The temperature of the exhaust gas from the warmed up engine is typically in the range of about 200-400 ° C. Although the practice of the present invention is illustrated in the case of a diesel engine, it is understood that the method can be used to treat the exhaust gas of a lean burn gasoline engine that uses excessive air to burn fuel or the exhaust gas of a power plant to which any hydrocarbons are supplied. Can be. In diesel engine exhaust, the hot gas also contains CO, CO ₂ , H ₂ O and incompletely burned hydrocarbons (somewhat in particulate form) in addition to O ₂ and N ₂ . However, the components of the exhaust gas applicable to the subject invention include nitrogen oxides (having trace amounts of N ₂ O) formed by the reaction of O ₂ and N ₂ in the combustion cylinder of the engine (or power plant). Predominantly NO and NO ₂ are collectively called NO _x ). The content of NO _x in diesel exhaust is typically about 200 to 300 parts per million (ppm). It is therefore an object of the present invention to treat nitrogen oxides which constitute about 2 to 3 / 10,000ths of the volume of the exhaust stream.

Referring again to FIG. 1, diesel engine 10 exhausts an exhaust stream, which first passes through dual bed catalytic oxidation reactor 14 and ultimately through catalytic reduction reactor 24. The exhaust stream is processed along its flow path and its composition is changed. The exhaust gas stream exiting the engine, represented by 12 in FIG. 1, passes through a catalytic oxidation reactor 14 where Pd is a suitable catalyst. Action of the oxidation catalyst reactor 14 is to substantially complete the oxidation of CO oxidation to CO _2, and CO ₂ and H ₂ O of the hydrocarbon containing fine particles. Some hydrocarbons can be converted to aldehydes through a partial oxidation process. Acetaldehyde (AA) is a representative aldehyde product. The oxidation reactor 14 does not reduce the amount of NO _x in the exhaust gas. The oxidized exhaust stream, shown at 16, is directed directly to the catalytic reduction reactor 24. As described below, the reduction catalyst applied to reactor 24 has been specifically developed for the reduction of NO _x under very lean conditions containing excess oxygen, the reaction being selective for lean NO _x . Often referred to as selective catalyst reduction (SCR). According to the invention, the diesel engine exhaust is treated with two separate side streams before being introduced into the reduction catalyst reactor 24.

A compressed air stream suitably drawn out of the engine compartment is an effective tubular non-thermal plasma reactor to convert some oxygen in the air into ozone and possibly other activated oxygen species. Blown through 20. The ozone containing plasma is added to the exhaust gas to oxidize NO to NO ₂ . Following the plasma addition, ethanol from the reservoir 26 is added separately to the diesel exhaust in the manner described below, wherein the ethanol acts as a reducing agent for NO ₂ . Residual ozone can oxidize some ethanol to acetaldehyde and also act as a reducing agent for NO and NO ₂ . In FIG. 1, after addition of the ozone-containing plasma, the exhaust stream is represented by 18 and exhaust gas stream after the ethanol-containing additive is added is represented by 28. The exhaust stream treated with both ozone and ethanol is introduced into the reduction reactor 24.

The catalytic reduction reactor 24 contains a dual bed reduction catalyst. The upstream catalyst bed comprises a predetermined volume of barium or sodium Y zeolite (denoted as BaY) and the downstream bed typically comprises copper Y zeolite (denoted as CuY) in a smaller volume. The reactor inlet or the temperature inside the reactor is used to adjust the plasma power density and the molar ratio of ethanol-to-NO _x for efficient operation of the catalytic reduction reactor 24. For example, the temperature at the inlet and outlet of the reduction catalyst can be monitored for efficient exhaust gas treatment by thermocouples (indicated by T1 and T2) or other suitable temperature sensor (s). Temperature data is sent to a digital controller (not shown) to adjust the plasma power density and the amount of ethanol added. The average temperature in reactor 24 is a function of the temperature of the exhaust gas introduced into the reactor and then the heat of reaction occurring in each of the beds. The upstream bed is mainly efficient at catalyst temperatures up to about 300 ° C., and the downstream bed is more efficient at higher catalyst temperatures. Stream 30 represents the treated exhaust gas exiting the exhaust system.

As shown in FIG. 2, a suitable non-thermal plasma reactor 20 includes a tubular dielectric body 40 in the form of a cylinder. The reactor 20 has two electrodes, a high voltage electrode 42 and a ground electrode 44, separated by a tubular dielectric body 40 and an air gap 46. The high voltage electrode 42 is a straight rod located along the longitudinal axis of the tube 40. The ground electrode 44 is a wire wound around the tubular dielectric body 40 in a spiral pattern. The helical ground electrode 44 coupled with the axial high voltage electrode 42 provides an active 54 and passive 52 electric field of helical regions intertwined with each other along the longitudinal direction of the reactor 20. The spirally active electric field 54 around the ground electrode 44 is highly focused for efficient plasma emission and ozone emission.

A high voltage, high frequency electrical potential is applied to the terminal leads 48 and 50 of the center electrode. The spiral outer ground electrode 44 is grounded as indicated by 56. During operation of the plasma reactor 20, air flows in one direction of the reactor around the center electrode 42, inside the dielectric tube 40, and the other end in the direction of the arrow shown in FIG. The potential applied to the center electrode 42 generates the above-described active 54 and passive 52 electric fields inside the reactor 20. These high voltage, high frequency electric fields 52, 54 generate reactive oxygen species within the air stream flowing in the air gap 46, which results in the production of ozone. This ozone containing air stream exits the reactor 20 and is immediately introduced into the exhaust stream as shown in FIG. As described in detail below, electrical power is applied to the plasma reactor 20 at a level that is an inverse function of the exhaust stream temperature. The side-stream plasma reactor 20 is located close to, but away from, the hot exhaust pipe.

As shown in FIG. 1, ethanol can be injected directly from the reservoir 26 into the exhaust stream and will evaporate quickly to aid in its reducing action. In addition, air may be bubbled through the reservoir 26 of ethanol dissolved in a low vapor pressure solvent, such as ethanol or diesel fuel, to entrain ethanol vapor and may be carried to the exhaust stream. As explained in more detail below, the amount of ethanol introduced into the exhaust stream depends on the NO _x content of the exhaust gas and the temperature of the reduction catalyst.

There are four important factors in the practice of the present invention: sidestream non-thermal plasma reactors for generating ozone from air; Composition of the reduction catalyst; The ethanol reducing additive; And management of operating parameters, in particular plasma energy and the feed ratio of said ethanol / NO _x . The plasma energy density and the ethanol / NO _x feed ratio vary with the reduction catalyst temperature. The exhaust gas treatment method of the present invention in combination with these four elements will be illustrated using an experimental setup and an engine dynamometer setup.

Experimental-Laboratory Setup

An experimental reactor system was prepared to evaluate the reduction of NO _x to N ₂ in the synthesis exhaust gas stream by the process illustrated by FIG. 1. The experimental catalytic reduction reactor 24 has a small upstream-downstream, dual-bed NaY zeolite / CuY zeolite catalytic reduction reactor. A facility was provided to provide a passage through synthetic NO and oxygen containing exhaust gases and the catalytic reduction reactor for treatment with plasma treated ozone and with ethanol. This experimental arrangement provides flexibility for the various flow arrangements of the gas stream through the plasma reactor and for the location of the ethanol introduced. Experimental conditions for the experimental reactor system are summarized in Table 1 below.

Table 1: Experimental conditions for experimental reactor system

Catalyst: NaY (9.9 wt% Na),

BaY (14.7 wt% Ba)

CuY (7.3 wt% Cu)

Feed Conc .: NO = 225 ppm

HC = 1200-4800 ppm C ₁

O ₂ = 16.8%

H ₂ O = 1.6%

N ₂ = remainder

Temperature: Plasma = 150 ° C

Catalyst = 150 ~ 400 ℃

Plasma: Power Density = 6 ~ 20J / L

AC voltage = +/- 7kV

Pressure: 101.3kPa

Space Velocity: Plasma = 25 K / h,

Catalyst = 11K ~ 44K / h (NaY, BaY)

22K ~ 44K / h (CuY)

For most of the tests, the flow rate of the synthetic exhaust gas containing 225 ppm of NO was 143 cm 3 / min. (STP) and ethanol addition rate were similarly 0.00051 cm 3 / min. The simulated exhaust gas was obtained by mixing the respective gases whose flow rate was controlled by mass flow controllers to obtain the desired exhaust gas composition. Water vapor was added to the gas mixture by passing the gas stream through a bubbler containing deionized water at room temperature. The gas mixture produced from the catalytic reduction reactor 24 (as in the FIG. 1 system) is reduced by an equal volume of N ₂ to suppress any condensation in the gas flow line between the reactor and the analyzers. Diluted. The chemical composition of the feed and output streams to and from the reactor was monitored by a FTIR gas analyzer (Nicolet Nexus 670). The temperature of the FTIR gas cell was maintained at 165 ° C. The NO _x conversion measured by the FTIR was checked by a chemiluminescent NO / NO _x analyzer. The two results agreed well within 5%.

The plasma reactor 20 used in the experimental test was made of a 3/8 inch outside diameter (o.d.) quartz tube with a high voltage electrode 42 in the center. Ground electrode 44 was a Ni-coated copper wire wound in two complete turns around the outer surface of the quartz tube with a pitch of 0.2 cm. The total length of the plasma region was 0.4 cm. The high voltage electrode 42 was made of a 1/16 inch od stainless rod inserted into an 1/8 inch od alumina tube, where air was passed through an annular air gap 46 to provide plasma Formed. For the purpose of comparison, sometimes simulated exhaust gas was passed through the plasma reactor 20 in place of air. A sinusoidal high voltage of +/- 7 kV at a frequency of 200 Hz was generated by a function generator (Tektronix Model AFG 310) and a high-voltage amplifier (Trek Model 20 / 20B), and a high-voltage probe (Tektronix Model P6015A). The electrical response of the plasma reactor 20 was monitored by a digital oscilloscope (Tektronix Model TDS 430A) and a computer, the digital oscilloscope including applied voltage, gas emission current, and power absorption by the plasma changing over time. The instantaneous response of the plasma reactor 20 was tracked while the computer showed the time-averaged power consumption by the plasma. For all reported experimental results, the inlet temperature of the plasma reactor 20 was maintained at 150 ° C. In practice, however, the plasma reactor 20 may be operated at room temperature.

The catalytic reactor 24 received a reduction catalyst having two separate catalytic reaction zones. In this experimental test, the synthesis exhaust was at room temperature and the catalyst was heated to a test temperature over a range of 150-400 ° C. The first catalyst zone comprised barium (or sodium) Y zeolite and occupied about four times the size of the second catalyst zone comprising copper Y zeolite. The following parallel chemistry was observed between the ethanol reducing agent and the NO ₂ on the BaY (or NaY) zeolite catalyst starting at temperatures below 300 ° C .:

EtOH + NO ₂ → AA + NO → N ₂

EtOH + NO ₂ → N ₂

Thereafter, as the exhaust gas passes over the CuY zeolite catalyst at a temperature above 300 ° C., the following reactions occur:

EtOH + NO + NO ₂ + O ₂ → N ₂

Two catalysts, BaY and CuY, were prepared by water-soluble ion-exchange of Ba (NO ₃ ) ₂ and Cu (NO ₃ ) ₂ and NaY, respectively. All samples in powder form were pelletized and crushed to a particle size of 30-40 mesh. Granulated catalyst was placed downstream of the plasma reactor 20 and filled with a tubular reactor made of 1/4 inch od quartz tube. Thereafter, the BaY (or NaY) catalyst was followed by the CuY catalyst. The catalysts were pretreated for 3 hours at 400 ° C. in a stream of air containing 2.5% water vapor. The reactor temperature was monitored at the reactor outlet 24 and controlled by an electronic temperature controller with an accuracy of about +/- 1 ° C.

The chemical composition of the exhaust gas was monitored at three different locations: before the plasma reactor, between the plasma reactor and the catalytic reactor, and after the catalyst reactor. A Motor Exhaust Gas Analyzer (Mexa-7500 DEGR) manufactured by Horiba was used for chemical analysis, including chemiluminescent NO / NO _x analyzers, HFID HC analyzers, O ₂ analyzers, and IR analyzers for CO, CO ₂ and N ₂ O were included. The HFID HC analyzer was used for total carbon from both HC and organic POHCs (partially oxidized hydrocarbons). Approximately 90% NO _x to N ₂ conversion was achieved using the exhaust gas treatment method of the present invention at a reactor outlet temperature of about 300 ° C.

Compared with conventional hydrocarbon reducing agents, the advantageous effect of using ethanol-containing reducing agents in combination with the reduction catalyst combination is shown in FIG. 3. Data was obtained from the exhaust stream to reduce NO ₂ to N ₂ using the BaY zeolite / CuY zeolite reduction catalyst combination. The test was carried out using three different reducing agents: propylene, gasoline and ethanol. The exhaust stream was passed through the reduction catalyst at a space velocity of 55000 / hr. On both the BaY zeolite catalyst and the CuY zeolite catalyst. The molar feed ratio (EtOH / NO _x ) of ethanol to NO _x was 18 and the plasma was generated with a plasma energy density of 20 J / L in the air stream. Each reducing agent was injected into the main stream line prior to the plasma reactor (pre-plasma injection). Denatured ethanol and E-85 (85% ethanol and 15% gasoline) were introduced using a syringe pump, while at the same time gasoline vapors were flowed by flowing air over the surface of the liquid gasoline contained in the quartz tube. Was introduced.

When each reducing agent was tested, the percent conversion of NO _x was plotted in minutes against the reduction catalyst operating time (“time-on-stream”). As shown in FIG. 3, higher conversion of NO _x was achieved over the operating time of the reduction catalyst when using ethanol as the reducing agent, while other reducing agents tested (propylene and gasoline) initially had higher NO _x. Conversion was achieved, but they quickly degraded the catalyst by coking. In the case of propylene, the NO _x conversion was increased during the initial about 20 minutes before the conversion began to decrease with a change in catalyst color from the original white to a light brown at the front of the catalyst bed. In the case of gasoline, the catalysts immediately began to lose their activity and the front of the catalyst bed turned grayish black. These results indicate that deactivation of the catalyst during the SCR process is due to the formation of coke on the catalyst surface. Interestingly, for ethanol, the catalytic activity improved with time-on-stream and the front of the catalyst bed remained the original index white. The catalyst deactivation does not occur when using ethanol as a reducing agent for NO _x reduction in an exhaust gas treatment system as ethanol and ethanol-containing HC (85% diesel fuel and E-diesel containing 15% ethanol, or 85%). Encouraging the use of ethanol and 15% gasoline).

Experimental tests were also performed to determine the preferred sequence for introducing the ozone containing plasma and ethanol into the NO _x containing exhaust gas stream. In this test, E-85 was injected using a syringe pump. In some tests, the E-85 was injected into the exhaust stream before the ozone containing plasma was introduced into the exhaust stream, presumably some ozone could react with ethanol. In another test, the ethanol was injected after an ozone containing plasma was introduced into the exhaust stream, so that only the simulated exhaust mixture was treated by the ozone containing plasma.

It was found that injecting E-85 before the plasma reactor produced more acetaldehyde (AA) but less NO ₂ than when E85 was introduced after the plasma reactor. The amount of AA produced was about 10% of the supplied ethanol, indicating low plasma efficiency to convert ethanol to AA. It was also concluded that the presence of ethanol in the exhaust stream inhibits the conversion of NO to NO ₂ by the plasma. Thus, this experimental test suggested that the ozone containing plasma is preferably introduced downstream of the NO _x exhaust gas stream.

to determine (a) the amount of ethanol required for catalytic reduction of the NO _x content in the catalyst temperature range over 150-400 ° C., and (b) the power required for the non-thermal plasma reactor over the same temperature range. Experimental tests were also performed. The data obtained in these tests is summarized in FIG. 4. The data in FIG. 4 shows the opposite trend of EtOH / NO _x feed ratio and plasma power density for optimized system performance over a BaY-CuY dual bed catalyst temperature range of 150-400 ° C. FIG.

The solid curve in FIG. 4 shows that at lower catalyst temperatures (eg, engine warm up temperatures), more plasma energy is required. The data requires that a power level of about 18 J / L is required over a catalyst temperature of 150-200 ° C. in this test with the synthetic exhaust gas, and then substantially reduced linearly, resulting in plasma power at about 400 ° C. Almost not required. Thus, it is shown that at these higher temperatures the plasma-free oxidation environment in the exhaust gas is suitable for oxidation of NO to NO ₂ for reduction to the present nonmetallic zeolite dual-bed catalyst.

This experimental test also shows that a larger molar ratio of ethanol to NO _x content is required with this increasing catalyst temperature. As shown in FIG. 4, the optimized molar feed ratio of ethanol to NO _x content gradually increases from about 5 to 25 over the catalyst temperature range of 150-400 ° C. This indicates that the amount of ethanol that is oxidized by oxygen in a more oxidizing atmosphere is increased before it is used as a reducing agent for NO _x .

In experimental tests under optimized operating conditions, a dual-bed catalytic reactor containing NaY and CuY exhibited 91% average NO _x conversion over a temperature range of 200-400 ° C. when ethanol (E85) was used as the reducing agent. Obtained. The simulated exhaust gas mixture contained 215 ppm of NO, its space velocity (SV) through the NaY bed was 11 K / hr, and its space velocity (SV) through the CuY bed was 22 K / hr. NO _x conversion (%) data over a temperature range of 150-400 ° C. is shown graphically in FIG. 5. The optimized operating conditions used in the experimental test were as follows; The feed ratio of hydrocarbon to nitrogen oxides (HC / NO _x ) varied with catalyst temperature (eg, 24 at 400 ° C., 12 at 300 ° C. and 6 at 200 ° C.), with a catalyst temperature range of 150-350 ° C. A plasma power constant of 12 J / L was maintained over and turned off at 400 ° C.

Experimental-Dynamometer Setup

The test was performed with an engine dynamometer with a 6 cylinder diesel engine with a 4.9 L displacement. The engine was operated under controlled conditions to produce exhaust gas for experimental treatment by the present process. This experimental setup is illustrated in FIG. 6. This allowed flexibility in the flow volume, flow direction and chemical analysis of the various flow streams. The volume of the diesel exhaust was greater than the volume of the catalytic reduction reactor provided in the experiments. Thus, only part of the exhaust gas was used for this test. Thus, the side stream line S was taken from the main exhaust line M and its flow rate was adjusted using a back-pressure valve as shown in FIG. 8. Since the side stream (S) was used in the following experiment. Engine specifications and experimental conditions for the dynamometer tests are summarized in Table 2.

Table 2. Representative Experimental Conditions for Engine Dynamometer Testing

Engine: 4.9L / 6 Cylinder Isuzu Engine

Speed = 2000 rpm

Torque = 60 Nm

EGR = 0

Fuel = Chevron ULS (0.2 ppm S)

Swedish (2 ppm S)

Amoco Premium (400 ppm S)

Exhaust gas temperature = 240 ℃

Exhaust Gas Composition:

HC = 180 ppm C ₁

NO _x = 235 ppm

O ₂ = 16.7%

CO ₂ = 3.3%

CO = 0.11%

Side stream plasma reactor:

Cylinder shape

SV = 60K / h

Power Density (E _p ) = 18 J / L

Temperature (T _p ) = 25 ° C

Side stream phase reduction catalytic reactor:

Catalyst = Monolith (NaY, BaY, CuY, DOC)

Reactor = 2 '' o.d. Stainless steel tube

Reducing Agent = E-Diesel, Ethanol

SV = 5K ~ 10K / h (NaY, BaY)

20K ~ 40K / h (CuY)

Temperature (T _c ) = 150 ~ 400 ℃

Exhaust side stream from the engine:

Flow rate = 10 ~ 20L / min

The diesel exhaust stream thus obtained first passed through a palladium-based diesel oxidation catalyst (DOC) and a ceramic filter before contacting the dual-bed catalyst (BaY + CuY). Analysis of the oxidized stream by analyzer (AN) showed that some acetaldehyde was formed during this oxidation. The acetaldehyde has been shown to be useful as a reducing agent for reducing NO _x to N ₂ . Both the hyperplasma reactor (HP) and the E-diesel reservoir (E) were located outside of the engine exhaust stream (S).

In order to achieve higher exhaust gas stream flow rates used for the engine dynamometer testing, larger plasma reactors (HP) were manufactured by keeping the shape and diameter the same and increasing the plasma forming area of the experimental reactor. That is, the ground electrode was made of Ni-coated copper wire wound at 20 complete turns around the outer surface of the quartz tube with a spacing of 0.2 cm, with the result that the total length of the plasma-forming region was 4 cm. The electrical frequency was also increased to 2000 Hz at +/- 7 kV. The ozone containing stream (SS) was injected into the exhaust side stream (S).

In treating the dynamometer exhaust gas, either ethanol evaporated from the pure ethanol or E-diesel mixture by air was injected into the exhaust stream. The E-diesel (solution of 85% diesel fuel and 15% ethanol) was kept in reservoir (E), and air was bubbled through the solution to separate substantially pure ethanol vapor from the high vapor pressure diesel fuel, It is delivered from the stream B to the exhaust stream S. Diesel fuel in the ethanol-containing E-diesel mixture acts as a carrier for delivering the reducing agent to the exhaust stream. In order to maintain high levels of NO _x conversion, the amount of ethanol required may be about 0.1-1.0% of the total exhaust stream composition.

It was found that the ethanol containing air stream exiting the reservoir at room temperature was saturated with ethanol vapor. In the experimental setup, the ethanol- after the ozone containing plasma was injected at a rate of 0.037 cm 3 / min based on an exhaust gas flow of 10 L / min in the side stream (S) with about 235 ppm NO _x in the exhaust. A containing air stream could be injected into the exhaust gas in the side stream S (stream B). For the purpose of the comparative experiments, a facility was also provided for introducing ethanol saturated air into the plasma reactor HP (stream A) along the air stream treated in HP.

The reduction catalyst was prepared starting with standard cordierite honeycomb bricks (5.66 inches diameter by 6 inches long) coated with NaY Johnson Matthey. A cylindrical core piece having a 1.875 inch diameter was drilled using a 2 inch hole drill. BaY and CuY versions of honeycomb bricks were prepared by water-soluble ion-exchange of NaY-coated bricks using Ba (NO ₃ ) ₂ and Cu (NO ₃ ) ₂ precursors, respectively. The brick was then calcined at 500 ° C. for 4 hours under atmospheric conditions. The honeycomb catalyst was coated with an insulation blanket before being inserted into a 2 inch od stainless steel reactor tube. In the case of the dual-bed reactor, the NaY-coated (or BaY-coated) brick is followed by the CuY-coated brick in the downstream. The reactor temperature was monitored at both the inlet and outlet of the reactor with a representative accuracy of +/- 1 ° C. This temperature varied with the temperature of the exhaust stream from the diesel engine.

The chemical composition of the exhaust gas was monitored by an analyzer (AN) at three different locations: before the ozone plasma was introduced from the plasma reactor, after the plasma was introduced (but before the catalytic reactor) and after the catalytic reactor. Is it. A vehicle exhaust gas analyzer (Mexa-7500 DEGR) manufactured by Horiba was used for the chemical analysis, which is a chemiluminescent NO / NO _x analyzer, HFID HC analyzer, O ₂ analyzer, and CO, CO ₂ and N ₂ O An IR analyzer was included. The HFID HC analyzer was used for total carbon from HC and organic POHCs. A NO _x to NO conversion of about 98% was achieved using the exhaust gas treatment method of the present invention at a reactor outlet temperature of about 300 ° C.

The effect of the ozone containing plasma (essential to the practice of the present invention) in carrying out the NO _x reduction method is shown in FIG. 7. After the exhaust gas passed through BaY zeolite monolith at a space velocity of 5000 / hr. And then CuY zeolite monolith at a space velocity of 20000 / hr., Data was obtained from stream (S). The engine exhaust gas was generated using Chevron ULS fuel and HC and CO were removed using a palladium oxidation catalyst (DOC).

As shown in FIG. 7, the conversion of NO _x to NO was plotted against the temperature of the dual bed catalyst in degrees Celsius. Solid lines indicate NO _x conversion using ozone-containing plasma (“plasma on”), which was generated using a plasma energy of 18 J / L. Dotted line represents NO _x conversion without plasma (“plasma off”). At conventional catalyst temperatures (about 300 ° C.), higher conversions of NO _x (about 98%) can be achieved in the plasma-on state, while lower conversions (about 78%) are achieved in the plasma-off state. It became. In the case of the plasma-on, the NO _x conversion decreased as the temperature increased beyond the threshold of 300 ° C., and the maximum conversion was achieved at the threshold. Although the conversion of NO _x using the plasma-off reduction method increases with increasing temperature, a high NO _x conversion was not obtained with the plasma-off as in the case of plasma-on of any temperature tested. As can be seen from the graph, the NO _x conversion improves with increasing catalyst temperature when the plasma power is turned off.

The effect of the gas space velocity in the reduction reactor in the NO _x conversion of the NO _x reduction system of the present invention was also tested, where NaY zeolite was used as the reduction catalyst. As the space velocity decreases over the temperature range of 200-400 ° C, NO _x conversion performance is improved and 90% conversion is achieved at 5000 / h and 200 ° C. For the space velocity of 10000 / h, the average rate of change of NO _x conversion with catalyst temperature was about 2 times higher than at 5000 / h, which is consistent with the kinetic theory. The experimental reactor data was then compared with the engine dynamometer test data, and the experimental setup used a NaY powder catalyst at a space velocity of 11000 / h. The performance of the honeycomb catalyst used in the dynamometer test with real engine exhaust at a space velocity of 5000 / h was about the same as that of the powder catalyst used in the experimental simulated exhaust stream at a space velocity of 11000 / h. . This indicates that a catalyst volume ratio of 2.2 can be applied between the powder catalyst and the honeycomb catalyst for the same performance. Therefore, the bulk volume of the honeycomb catalyst must be about two times larger than the powder catalyst to maintain the same performance.

8 compares the efficiency of two different E-diesel injection modes for performing NO _x conversion: before and after plasma injection. Prior to plasma injection, the ethanol is injected into the air stream (stream A in FIG. 8) and introduced into HP, thus treated in the hyperplasma reactor as part of the SS stream and introduced into the exhaust gas. After plasma injection, the ethanol was injected into stream SS, downstream of the plasma injection (stream B of FIG. 8). As shown in FIG. 8, after plasma injection of the E-diesel, slightly better NO _x conversion is achieved than before the plasma injection at both 200 and 300 ° C.

As another method of testing the efficiency of the catalyst, the temperature change of the catalyst bed was compared as follows. First, the system after plasma injection of E-diesel (plasma + catalyst) was stabilized until it reached a stable state, so that the temperature at the catalyst outlet was at a desired temperature (eg 200 or 300 ° C.). maintain. The E-diesel injection mode was then switched to the pre-plasma injection mode and at the same time the temperature change due to this switch from after the plasma injection to before the plasma injection was monitored. At either 200 or 300 ° C., with the increase of CO and CO ₂ levels, there was a significant temperature rise due to this switch. This represents an improved oxidation (combustion) of the E-diesel before the plasma injection as compared to after the plasma injection, which leads to a reduced NO _x conversion performance, albeit a small difference. Based on two observations of the experimental test and the dynamometer test, it can be concluded that the ethanol-based reducing agent does not require treatment with a plasma for efficient NO _x reduction. Preferably, ethanol was separated in the required amount and introduced downstream of the ozone-containing plasma injection.

The method of the present invention can be applied to the use of any type of fuel used in diesel engines despite the presence of sulfur in the fuel. Experiments were performed using three different fuels, including Chevron ULS, Swedish and Amoco premium, with 0.2 ppm S, 2 ppm S and 400 ppm S content, respectively. At a representative catalyst temperature of 300 ° C., the presence of S content in the fuel does not significantly affect the NO _x conversion.

Although the present invention has been described for the purpose of illustration, it is not intended to be limited by this detailed description, but only within the scope of the following claims.

Claims (14)

Passing air through a non-thermal plasma reactor to generate an ozone containing plasma and adding the plasma to the exhaust stream for oxidation of NO to NO ₂ ; Adding ethanol to the exhaust stream separately from the addition of the plasma for the reduction of the nitrogen oxides; And Then reducing the nitrogen oxides to N ₂ by contacting the exhaust stream with a dual bed reduction catalyst comprising NaY zeolite and / or BaY zeolite in a first bed and CuY zeolite in a second bed. A process for reducing NO-containing nitrogen oxides in an exhaust stream comprising oxygen, carbon monoxide and hydrocarbons at temperatures in excess of 150 ° C. 2. The method of claim 1 wherein ethanol is added downstream of the exhaust stream to which the ozone containing plasma is added. The reactor of claim 1, wherein the plasma reactor is a tubular vessel having a reactor space for flow through an air passage and spirally wound around the tubular vessel and the high voltage electrode located in the reactor space. And a ground electrode, thereby providing entangled spiral passive and active electric fields for generation of said ozone containing plasma. The method of claim 1, wherein the flow of the exhaust gas passing through the first bed is a space velocity greater than the flow space velocity of the exhaust gas passing through the second bed. 2. The method of claim 1, comprising adding ethanol to the exhaust stream as ethanol vapor in an air stream. 6. The method of claim 5, comprising passing an air stream through the ethanol solution to produce ethanol vapor in the air stream. 7. The method of claim 6, wherein the ethanol is dissolved in gasoline and comprises at least about 85% of the volume of the solution. The method of claim 6, wherein the ethanol is dissolved in diesel fuel. The method of claim 1, comprising operating the plasma reactor at a plasma energy density inversely proportional to the catalyst temperature at a catalyst temperature in the range of 150-400 ° C., wherein the plasma energy density is zero or zero at a catalyst temperature of 350 ° C. or higher. Reduction of nitrogen oxides to be reduced. The method of reducing nitrogen oxides according to claim 1, wherein ethanol is added to said exhaust stream in a molar ratio of ethanol to nitrogen oxides (EtOH / NO _x ) in the range of 5 to 25 as a function of catalyst temperature in the range of 150 to 400 ° C. . (a) an exhaust stream is passed through contact with an oxidation catalyst to oxidize carbon monoxide and hydrocarbons; (b) air passes through a non-thermal plasma reactor to generate an ozone containing plasma, add the plasma to the exhaust stream for oxidation of NO to NO ₂ , and reduce energy to zero at temperatures above 350 ° C. Accordingly, the energy applied to the plasma reactor is inversely proportional to the reduction catalyst temperature at a temperature in the range of about 150-400 ° C .; (c) add ethanol to the exhaust stream for the reduction of the nitrogen oxides, the amount of ethanol increases in proportion to the reduction catalyst temperature; And after that (d) flowing said exhaust stream through a dual-bed catalytic reduction reactor, said reactor comprising a first bed of barium and / or sodium Y zeolite catalyst and a second bed of copper Y zeolite catalyst And the volume of the first bed is greater than the volume of the second bed, wherein the NO-containing nitrogen oxide in the diesel engine exhaust stream comprises oxygen, carbon monoxide and hydrocarbons at a temperature in excess of about 150 ° C. 12. The reactor of claim 11 wherein the plasma reactor is a tubular vessel having a reactor space therein for flow through a passageway and spiraling around the tubular vessel and the high voltage electrode located in the reactor space. A method of reducing nitrogen oxides, comprising: a grounding electrode wound around, thereby providing helical passive and active electric fields intertwined for the generation of said ozone containing plasma. 12. The method of claim 11, comprising evaporating ethanol from E-diesel and adding the ethanol to the exhaust stream as a vapor in an air stream. 12. The process of claim 11 wherein ethanol is added to the exhaust stream in a molar ratio of ethanol to nitrogen oxides (EtOH / NO _x ) in the range 5-25 as a function of catalyst temperature in the range of 150-400 ° C. .

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